A conventional shadow-mask-type CRT comprises an evacuated envelope having therein a viewing screen comprising an array of phosphor elements of three different emission colors arranged in a cyclic order, means for producing three convergent electron beams directed towards the screen, and a color selection structure or shadow mask comprising a thin multiapertured sheet of metal precisely disposed between the screen and the beam-producing means. The apertured metal sheet shadows the screen, and the differences in convergence angles permit the transmitted portions of each beam to selectively excite phosphor elements of the desired emission color. A matrix of light-absorptive material surrounds the phosphor elements.
In one prior process for forming each array of phosphor elements on a viewing faceplate of the CRT, the inner surface of the faceplate is coated with a slurry of a photosensitive binder and phosphor particles adapted to emit light of one of the three emission colors. The slurry is dried to form a coating, and a light field is projected, from a source, through the apertures in the shadow mask and onto the dried coating so that the shadow mask functions as a photographic master. The exposed coating is subsequently developed to produce the first color-emitting phosphor elements. The process is repeated for the second and third color-emitting phosphor elements, utilizing the same shadow mask, but repositioning the light source for each exposure. Each position of the light source approximates the convergence angle of one of the electron beams which excites the respective color-emitting phosphor elements. A more complete description of this process, known as the photolithographic wet process, can be found in U.S. Pat. No. 2,625,734, issued to H. B. Law on Jan. 20, 1953.
A drawback of the above-described wet process is that it may not be capable of meeting the higher resolution demands of the next generation of entertainment devices and the even higher resolution requirements for monitors, work stations and applications requiring color alpha-numeric text. AdditionallY, the wet photolithographic process (including matrix processing) requires 182 major processing steps, necessitates extensive plumbing and the use of clean water, requires phosphor salvage and reclamation and utilizes large quantities of electrical energy for exposing and drying the phosphor materials.
U.S. Pat. No. 3,475,169, issued to H. G. Lange on Oct. 28, 1969, discloses a process for electrophotographically screening color cathode-ray tubes. The inner surface of the faceplate of the CRT is coated with a volatilizable conductive material and then overcoated with a layer of volatilizable photoconductive material. The photoconductive layer is then uniformly charged, selectively exposed with light through the shadow mask to establish a latent charge image, and developed using a high molecular weight carrier liquid. The carrier liquid bears, in suspension, a quantity of phosphor particles of a given emissive color that are selectively deposited onto suitably charged areas of the photoconductive layer to develop the latent image. The charging, exposing and deposition process is repeated for each of the three color-emissive phosphors, i.e., green, blue, and red, of the screen. An improvement in electrophotographic screening is described in U.S. Pat. No. 4,448,866, issued to H. G. Olieslagers et al. on May 15, 1984. In that patent, phosphor particle adhesion is said to be increased by uniformly exposing, with light, the portions of the photoconductive layer lying between adjacent portions of the deposited pattern of phosphor particles after each deposition step so as to reduce or discharge any residual charge and to permit a more uniform recharging of the photoconductor for subsequent depositions. Because the latter two patents disclose an electrophotographic process that is in essence, a wet process, many of the drawbacks described above, with respect to the wet photolithographic process of U.S Pat. No. 2,625,734, also are applicable to the wet electrophotographic process.
U.S. patent application Ser. No. 287,356, filed by P. Datta et al. on Dec. 21, 1988 now U.S. Pat. No. 4,921,767, describes an improved process for manufacturing CRT screen assemblies using triboelectrically charged dry-powdered screen structure materials and surface-treated carrier beads having a coupling agent thereon to control the polarity and magnitude of the imparted charge. Applicants have determined that, while CRT viewing screens can be electrophotographically manufactured using untreated phosphor particles, surface treatment of the phosphor particles increases the triboelectric charge on the phosphor particles, thereby causing a greater quantity of phosphor particles to be attached to each carrier bead. This improves the efficiency of the dry electrophotographic process and increases, by a factor of about 2 to 9 times, the screen weight for screens manufactured using surface-treated phosphors.